The present invention relates generally to image forming devices capable of forming text, graphics and/or other symbols on both sides of a media sheet. Specifically, the present invention relates to a method for adjusting the rotational skewing of text, graphics and/or other symbols to be formed on opposing sides of a media sheet relative to one another, as well as relative to the edges of the media sheet.
Image forming devices, such as laser printers, inkjet printers, copiers, facsimile devices, and the like, adapted for forming text, graphics and/or other symbolsxe2x80x94which will be referred to herein collectively as simply xe2x80x9cimagesxe2x80x9dxe2x80x94on a media sheet are well known in the art. These image forming devices typically apply ink or toner to the media sheetxe2x80x94for example, a pre-cut paper sheetxe2x80x94in a specified pattern to form the images thereon. Such devices may also be adapted to form images on both of the opposing sides of a media sheet, the process of forming images on opposing sides of a media sheet commonly being referred to as duplex printing. The advantages of duplex printing include reducing the quantity of paper required for a print set as compared to one-sided (simplex) printing, and generating print sets with layouts resembling that of professionally printed books.
Conventional image forming devices adapted for duplex printing, such as duplex printers, typically include a source device, such as a paper tray, and a feed mechanism or pick roller to draw an individual media sheet from the source device and to feed the media sheet to a print engine. The print engine is configured to receive the media sheet and to form images on one side thereof To form images on the media sheet, the print engine includes a printing apparatus, wherein the printing apparatus may be of the ink jet type or the electrophotographic type (i.e., a laser printer), or any other suitable printing apparatus known in the art. Subsequently, the media sheetxe2x80x94having images formed on one side thereofxe2x80x94is transferred to a path selection gate. If simplex printing is desired, the path selection gate directs the media sheet to an output device, such as an output tray, where the media sheet is retained for pick-up by an operator. If duplex printing is desired, the path selection gate routes the media sheet to a duplexer. The duplexer is an apparatus configured to enable printing of a second, opposing side of the media sheet by the print engine.
In one type of duplexer known in the art, the path selection gate directs the media sheet to a secondary tray where the media sheet is temporarily held. Once all of the media sheets in a print set have received the appropriate images on a first side thereof, respectively, the media sheets are drawn out of the secondary tray and are fed back to the print engine for application of images to a second, opposing side thereof, respectively. In an alternative approach, the duplexer comprises a duplexing paper path configured to directly reverse and return a media sheet (having images applied to one side thereof) to the print engine for application of images to a second side thereof. Operation of the pick roller, print engine, path selection gate, and duplexer is typically controlled by a controller. The controller typically includes firmware, which stores data and routines that enable operation of the image forming device. Also, an image forming device usually includes a control panel for inputting data and/or commands into the controller.
As a media sheet travels through an image forming device, it may become misaligned relative to the print engine, resulting in application of images to the media sheet that are skewed relative to the media sheet, as well as resulting in images on one side of a media sheet that are skewed relative to images on a second, opposing side of the media sheet. Generally, there are two types of skewing, including translational skew and rotational skew. Translational skew occurs when images are offset horizontally and/or vertically from a desired location on a media sheet, and/or when images on one side of a media sheet are horizontally and/or vertically offset from images on a second, opposing side thereof. Rotational skew occurs when images are rotationally offset through an angle from a desired rotational orientation on a media sheet, and/or when images on one side of a media sheet are rotationally offset through an angle relative to the rotational orientation of images on a second, opposing side thereof.
Skew, whether translational or rotational, can result from any one of a number of circumstances, or a combination thereof For example, a media sheet may be misaligned in the source device and this misalignment may not be corrected as the media sheet is drawn out of the source device. Misalignment in the source device may result from wear and tear of the source device and/or from a source device or component thereof that fails to meet design tolerance specifications. Misalignment may also be imparted to a media sheet as it travels from the source tray to the print engine and/or as it travels through a duplexer and back to the print engine. Typically, an image forming device includes a plurality of rollers and guides, as well as one or more actuators, configured to move media sheets through the image forming device and between the pick roller, print engine, path selection gate, and duplexer. Due to extended use, these mechanical parts may become worn or damaged and fail to operate effectively. Also, a roller, guide, actuator, or other mechanical part, may not function properly due to manufacturing defects and/or a failure to meet design tolerance specifications.
The condition of a media sheet itself may affect the manner in which an image forming device is able to manipulate that media sheet. For example, variation in media sheet dimensions may result in skewing, especially in duplex printing wherein both opposing edges of a media sheet may be used for alignment or registration. Variation in media sheet dimensions may be the inherent result of manufacturing processes or such variation may be introduced by a printing process. For instance, electrophotographic printing generally involves passing a media sheet through a heated fuser in order to create a permanent image. Passing the media sheet through a heated fuser may reduce the moisture content of the media sheet and thereby alter one or more of the dimensions of the media sheet during printing. Moreover, the moisture content of a media sheet before printing may affect manipulation of the media sheet by an image forming device. For example, a very high moisture content (i.e., as may be found in a humid climate) or a very low moisture content (i.e., as may be found in a dry climate) of a media sheet may impact the ability of that media sheet to be manipulated by an image forming device, thereby resulting in skew.
It is known in the art to perform duplex registration to compensate for translational skew. Duplex registration generally comprises adjusting the horizontal and/or vertical positions of images on one or both sides of a media sheet to align these images relative to one another in the horizontal and vertical directions. One method for performing duplex registration of an image forming devicexe2x80x94such as described in U.S. Pat. No. 6,118,950 to Wibbels et al.xe2x80x94includes printing a test page having, on one side, a first set of horizontal and vertical demarcation elements (i.e., a first portion of a vernier scale) and, on an opposing side, a second set of horizontal and vertical demarcation elements (i.e., a second portion of a vernier scale). The test page can be held up to a light, enabling an operator to compare the second set of demarcation elements against the first set of demarcation elements and, thus, providing the operator with a means for determining horizontal and vertical correction values for images on one or both sides of a media sheet. These horizontal and vertical correction values are then input via a control panel to the image forming device""s controller, which then directs the print engine to reposition the images on one or both sides of the media sheet in accordance with the respective correction values, such that the images on the opposing sides of a media sheet line up with one another, as well as with the edges of the media sheet, in the horizontal and vertical directions. However, the above-described duplex registration procedure ignores rotational skewing problems.
As printers are being designed to operate at ever-increasing speeds and greater duty cycles, problems such as misalignment and skew resulting from extended use and wear become even more significant. Thus, especially for duplex printing, the failure to compensate for rotational skewing may result in poor quality duplex images. Further, even though an image forming device may be outputting duplex images that meet performance specifications, it may still be desirable to compensate for rotational skewing, as well as for translational skewing, in order to meet user demands in certain high performance markets, such as book printing. Accordingly, a need exists in the art for methods and apparatus for compensating for rotational skew in duplex printing operations.
The present invention encompasses methods and apparatus for reducing or eliminating rotational skew between images formed on opposing sides of a media sheet. Image forming devices capable of duplex printingxe2x80x94i.e., capable of applying text, graphics, and/or other symbols on both of the opposing sides of a media sheetxe2x80x94are well known in the art. Such imaging devices typically comprise a print engine adapted to form images on a media sheet received from a source device, which may include one or more source trays or any other suitable media source. Conventional image forming devices further include a path selection gate configured, for simplex printing, to direct a media sheet having images on one side thereof to an output device and, for duplex printing, to direct a media sheet to a duplexer configured to enable printing of a second, opposing side of the media sheet by the print engine. Due to extended use and wear of the various mechanical and electromechanical components comprising such an image forming device, as well as a failure to adhere to design tolerances and the presence of manufacturing defects in these components, misalignment often occurs between a media sheet moving through an image forming device and the print engine, resulting in rotational skew between images formed on opposing sides of the media sheet.
Rotational skew, as noted above, is reduced or eliminated using a duplex test page having a skewing scale according to the invention. The skewing scale comprises a first portion formed on one side of the duplex test page and a second portion formed on a second, opposing side of the duplex test page. The first portion includes mutually orthogonal coordinate axes intersecting at an origin and a skew axis intersecting one of the coordinate axes and oriented perpendicular thereto. The skew axis includes a plurality of tick marks spaced along its length, each tick mark corresponding to a rotational skew correction factor, which may be printed next to its corresponding tick mark. The second portion of the skewing scale comprises a substantially straight line, or skew line, that intersects the origin of the first portion. This skew line is hypothetically parallel to one of the coordinate axes of the first portion (and perpendicular to the other of the coordinate axes).
The first portion of the skewing scale in cooperation with the second portion of the skewing scale provide a tool for compensating for rotational skew. If misalignment exists between the duplex test page and the print engine of an image forming device, the skew line will be rotationally skewed relative to the first portion of the skewing scale formed on the first side of the duplex test page. To compensate for this rotational skew, the first portion of the skewing scale is compared against the second portion, and a rotational skew correction factor is determined from this comparison. In particular, the skew line of the second portion is compared to the coordinate axes and skew axis of the first portion, and the tick mark most closely spaced to the point of intersection between the skew line and the skew axis corresponds to the required rotational skew correction factor, which may be directly read from the duplex test page. Comparison between the first and second portions of the skewing scale may be facilitated by holding the duplex test page up to a light source.
The rotational skew correction factor can then be provided to the image forming device, and a controller of the image forming device directs the print engine to alter placement of images on the first and/or second side of the media sheet based upon the rotational skew correction factor, thereby rotationally aligning images on opposing sides of a media sheet relative to one another. Altering placement of images on one or both sides of a media sheet may be achieved using coordinate transformation matrices. Rotational skew correction factors may also be provided to the image forming device to orient images formed on the opposing sides of a media sheet relative to the edges of the media sheet itself. In an alternative embodiment, a unique rotational skew correction factor is selected according to the invention for each source tray of an image forming device having two or more source trays.
The selection of a set of rotational skew correction factors is arbitrary, and the only requirement is that the image forming device be capable of interpreting a rotational skew correction factor and performing the necessary adjustments. Also, although the tick marks on the skew axis may be equidistantly spaced, in an alternative embodiment the distances between adjacent tick marks are not all the same. Further, the distance between two adjacent tick marks does not necessarily have to correspond to a physical parameter, such as a measured angle. In another alternative embodiment, however, the distances between adjacent tick marks correspond to a standard unit angle as measured about the origin of the coordinate axes.
Another embodiment of the invention comprises an image forming device, such as an ink jet printer, laser printer, photo copier, or facsimile, configured to generate and print a duplex test page, as described above. The image forming device is further configured to receive a rotational skew correction factor and to perform the appropriate corrective action in response to the rotational skew correction factor. The data, routines, and/or executable instructions enabling the printing of a duplex test page and enabling the image forming device to receive and respond to a rotational skew correction factor may be included in the firmware of a controller. Also, an image forming device may be configured to receive and store a unique rotational skew correction factor for each source tray of an image forming device having a source device including multiple source trays.